Current limiting fuse

ABSTRACT

A current limiting fuse structure comprising a generally tubular electrically insulating casing having terminal means disposed adjacent to the opposite ends thereof. One or more fusible elements or links is connected between the terminal means. An electrically insulating support member on which each fusible element is disposed is positioned within the associated casing and extends axially between the respective terminal means. At least the axially intermediate portion of the support member is formed from a material which evolves one or more gases which assist or aid in arc extinction in the presence of the arc which results when an associated fusible element melts. The lastmentioned material is also electrically non-tracking in the presence of an arc.

United States Patent 1 1111 3,742,415

Cameron et al. 1 June 26, 1973 CURRENT LIMITING FUSE Primary Examiner-Harold Broome [75] Inventors: Frank L. Cameron, Irwin; Harold L. Assistant g 'qf g i 1 Miller, West Elizabeth, both of Pa. Atwmey mmon at a [73] Assignee: Westinghouse Electric Corporation,

Pittsburgh, Pa. [57] ABSTRACT Filedi P 1971 A current limiting fuse structure comprising a generally [211 App]. No; 185,201 tubular electrically insulating casing having terminal means disposed adjacent to the opposite ends thereof. One or more fusible elements or links is connected be [52] US. Cl.. 337/279, 337/159 tween h i l mean5 An electrically insulating [51] Int. Cl. H0lh 85/44 Suppon member on which each f ibl demem is [58] Field of Search 337/159, 160, I61, posed is positioned within the associated casing d 337/273, 279 tends axially between the respective terminal means.

At least the axially intermediate portion of the support 1 References Cited member is formed from a material which evolves one UNlTED STATES PATENTS or more gases which assist or aid in arc extinction in the 2,917,605 12/1959 Fahnoe 337/159 Presence of the are which results when an associated 3,287,525 11/1966 Miku| k 337/161 fusible element melts. The last-mentioned material is 2,143,031 1/1939 Rapp 337/279 also electrically non-tracking in the presence of an arc. 2,209,823 7/1940 Lohausen 337/293 X 10 Claims, 6 Drawing Figures arr CURRENT LIMITING FUSE BACKGROUND OF THE INVENTION In the construction of current limiting fuses, the cross-sectional size of the fusible elements employed is normally relatively small and each fusible element or link usually includes portions of reduced cross-section to provide the desired current limiting action. In order to support the fusible elements within the associated casing or housing in such current limiting fuses, an electrically insulating support member may be provided which is disposed within the associated casing or housing and which extends axially between the associated end terminals, such as disclosed in US. Pat. No. 3,294,936 or in copending application Ser. No. 872,895, filed Oct. 31, 1969 by Frank L. Cameron which is assigned to the same assignee as the present application.

In certain applications of current limiting fuses, such as at relatively high voltages which may be 5 RV and above, current limiting fuses may be required to interrupt overload currents over a full or relatively wide range which extends from relatively low overload currents to relatively high overload currents. In such applications, certain problems arise if the support member on which each fusible element is mounted is formed from an organic material, such as a glass reinforced polyester material, which evolves one or more gases in the presence of an arc to assist in arc extinction or interruption during certain operating conditions. These problems result because an axially extending continuous current leakage path may result along such a support member if the support member includes organic materials of the type which have been employed in the past. It is therefore desirable to provide an improved current limiting fuse structure of the type described which is uniquely adapted to relatively high voltage applications, as previously mentioned, and which is capable of operating over a relatively wide range of overload currents.

SUMMARY OF THE INVENTION In accordance with the invention, a current limiting fuse structure includes a generally tubular, electrically insulating casing or housing having terminal means mounted on said casing adjacent to each of the opposite ends of the casing. One or more fusible elements is disposed in the casing and electrically connected between the associated terminal means. In order to support each fusible element, an electrically insulating support member is disposed in the casing and extends axially between the associated respective terminal means. At least the axially intermediate portion of the support member is formed from a normally solid mate-. rial, such as compressed boric acid material, which is adapted to evolve one or more gases which aid in are extinction in the presence of the are which results when an associated fusible element melts or fuses. The lastmentioned material is substantially non-tracking in the presence of an arc to prevent the formation of an axially extending current leakage path along the support member during any interrupting operation of the overall fuse structure. To additionally aid in arc extinction or interruption, a quantity of pulverulent arc quenching material, such as quartz or silica sand, may be disposed. in or to substantially fill the casing in contact with or to embed each associated fusible element. Where the support member is generally hollow cylindrical in configuration, the inner bore of the support member may also be substantially filled with the same are quenching material.

Where desired, each fusible element provided may be formed from electrically conducting fusible material of the flat ribbon type having periodically spaced restricted or reduced portions along its length with the fusible element being helically wound or disposed on the associated support member. In the latter embodiment of the invention, each fusible element may be of an overall predetermined length with each of the end portions being formed by bending or doubling a predetermined length of each of respective ends of the fusible material back on itself to increase the effective cross-sectional area of each of said end portions to be relatively greater than that of the associated intermediate portion of the fusible element.

In another embodiment of the invention, only the axially intermediate portion of the support member is formed from a normally solid material which evolves one or more gases which assist or aid in arc extinction in the presence of an arc and which is substantially nontracking in the presence of an arc. In this embodiment of the invention, the axially end portions of the support member may be formed from an electrically insulating material which does not evolve gases in the presence of an arc and which is substantially non-tracking in the presence of an arc, such as a ceramic or other inorganic material, and which can be formed into a solid member or part. More specifically, in such an embodiment, the support member may include a solid central supporting rod which extends axially between the respective associated terminal means of the current limiting fuse structure and a plurality of generally tubular or hollow cylindrical, normally solid members which surround the associated rod and extend axially along with the associated rod to form the different portions of the support member described above.

BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of the invention, reference may be had to the preferred embodiment exemplary of the invention shown in the accompanying drawings in which:

FIG. 1 is a view, partly in elevation and partly in section, of a current limiting fuse structure embodying the invention;

FIG. 2 is a top plan view of the fuse structure shown in FIG. I with the upper end cap or ferrule removed;

FIG. 3 is an elevational view of a subassembly which forms part of the fuse structure shown in FIGS. 1 and 2 and which includes a fusible element and an associated electrically insulating support member on which at least a portion of the fusible element is disposed;

FIG. 4 is an elevational view of the electrically insulating support member which forms part of the subassembly shown in FIG. 3 and, in turn, part of the fuse structure shown in FIGS. 1 and 2;

FIG. 5 is a top plan view of the electrically insulating support member shown in FIG. 4; and

FIG. 6 is an elevational view of a second embodiment of an electrically insulating support member which may form part of the fuse structure shown in FIG. 1 and which may be employed instead of the support member shown in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings and FIG. I in particular, there is illustrated a current limiting fuse structure 10 which is particularly adapted for high voltage applications, such as 5 KV and above, and which embodies the principal features of the invention. As illustrated, the fuse structure includes a generally tubular casing or housing which is formed from a suitable electrically insulating material which has sufficient structural strength to withstand the thermal conditions and internal pressures which may result during the operation of the fuse structure 10, such as a glass-reinforced melamine or epoxy resin material. In order to close off the opposite ends of the casing 20 and to provide means for making electrical connections to the fuse structure 10 adjacent to the ends thereof, the terminal end caps or electrically conducting ferrules 82 and 84 are secured to the opposite ends of the casing 20 by suitable means, such as the magnetic forming method which is described in detail in U.S. Pat. No. 3,333,336 which issues Aug. 1, 1967 to F. L. Cameron and W. C. Good and which is assigned to the same assignee as the present application. Where desired, the axially projecting, electrically conducting studs 92 and 94 may be mounted on or integrally formed with the end caps 82 and 84, respectively, in order to permit the mounting of the fuse structure 10 in particular types of supporting structures. As illustrated, the fuse structure 10 also includes the internal terminal members 72 and 74 which are generally disposed between the opposite ends of the casing 20 and the respective end caps 82 and 84. Each of the terminal members72 and 74 is formed from a suitable electrically conducting material, such as copper or a copper alloy, and as illustrated includes a central opening with a pair of tab portions formed integrally at the opposite sides of the central opening which project axially inwardly at one end of the associated casing 20, as'indicated at 72A and 72B for the terminal member 72 and as indicated at 74A and 748 for the terminal member 74, as shown in FIG. ll.

In order to assist in supporting the fusible element 60 in a desired assembled configuration or disposition which purpose be described in detail hereinafter, to assist in properly positioning the fusible element 60 inside the casing 20 at a location which is laterally or radially spaced from the inner bore of the casing 20 and for other purposes which will be described hereinafter, the electrically insulating support member 30 is disposed inside the casing 20 and extendsaxially from the terminal member 72 at one end of the casing 20 to the terminal member 74 at the other end of the casing 20. More specifically, as illustrated in FIGS. 4 and 5, the elongated support member 30 is generally tubular or hollow cylindrical in configuration and includes a central bore as indicated at 30A which extends axially between the opposite ends of the support member 30. As shown, the support member 30 includes a plurality of generally annular blocks or generally tubular members which are assembled or stacked in end to end relation along a common axis with the meeting ends of the respective blocks 31, 32, 33, 34, 35, 36, 37, 38 and 39 being structurally joined or bonded to one another by a suitable electrically insulating bonding material, such as an epoxy resin material. It is to be noted that each of the end blocks or portions 31 and 39 includes a pair of flat tened sides, as indicated at 31A and 318 for the upper block 31 which are adapted to be received by the tab portions of the respective terminal members 72 and 74 to assist in retaining the support member 30 in the desired central position inside the associated casing 20 when the support member 30 is assembled inside the casing 20, as shown in FIG. ll. It is also to be noted that each of the blocks 3i through 39 is generally circular in cross-section.

As illustrated, the support member 30 and more specifically the plurality of blocks 31 through 39 which make up the support member 30 is preferably formed from an electrically insulating, normally solid material which is adapted to evolve one or more gases which aid in arc extinction or interruption in the presence of one or more arcs which results when the associated fusible element melts or fuses and which is substantially nontracking in the presence of an are which results during the operation of the fuse structure 10. An example of such material which has been found to be particularly effective in practicing the disclosed invention is highly compressed boric acid material which evolves water vapor when exposed to an arc during the operation of the fuse structure 10. The desired density-of the compressed boric acid material from which the blocks 31 through 39 may be formed may be obtained by compressing the boric acid material until the minimum hardness of the boric acid material is about 35 on the D scale of a well-known hardness measuring instrument which is sold under the trade name Shore Durometer.

' To achieve such a hardness of the boric acid material requires that the boric acid material be compressed by a minimum isostatic pressure of approximately 25,000 pounds per square inch. It is to be understood that where desired, the entire support member 30 may be formed from a suitable material, such as compressed boric acid, as a single unitary member rather than forming the support member as previously described from a plurality of separate blocks which are structurally joined or bonded together.

In general, the fuse structure 10 includes one or more fusible elements or links 60 which are electrically connected and extend axially between the terminal end caps 82 and 84 or between the terminal members 72 and 74, as shown in FIG. 1. As illustrated the fuse structure It) includes a single fusible element 60 which is electrically connected between the terminal members 72 and 74. More specifically, as best shown in FIG. 3, the fusible element 60 comprises the first and second end portions 60A and 60C and an intermediate portion 608. As shown in FIG. 3, the fusible element 60 may be formed from a predetermined length of electrically conducting fusible material, such as silver, of the flat ribbon type and include a plurality of axially spaced points of reduced or restricted cross-sectional area, as indicated atotlD, which may be formed by V-notching the ribbon material from which the fusible element 60 is formed on both sides at spaced points along its length. The construction of the fusible element 60 results in a series of restricted areas which fuse initially during an interrupting operation of the fuse structure 10 to provide a series of axially spaced arcs, the sum of the voltages across said arcs resulting in a relatively high total arc voltage during the operation of the fuse structure E0 to limit the overload current which flows to a value less than that which would otherwise result.

As also shown in FIG. 3, the fusible element 60 includes a ball or bead of M effect causing material or means, as indicated at 65, which comprises a quantity of low melting point metallic alloy, such as a combination of tin and lead. The effective cross-sectional area of each of the end portions 60A and 60C of the fusible element 60 may be increased compared with that of the intermediate portion 60B by bending each of the respective ends of the predetermined length of fusible material from which fusible element 60 is formed back on itself a predetermined distance or length to form the end portions 60A and 60C. The cross-sectional area of each of the end portions 60A and 60C is substantially twice as great as the cross-sectional area of the intermediate portion 608 of the fusible element 60.

After the fusible element 60 is formed as shown in FIG. 3, the intermediate portion 608 may be helically wound or assembled on the associated support member 30 with the successive turns of the fusible element 60 being axially spaced from one another, as shown in FIG. 3. In order to secure the intermediate portion 60B of the fusible element 60 to the support member 30 after the intermediate portion 60B is assembled on the support member 30, as shown in FIG. 3, suitable means, such as a pair of fastening tapes 52 and 54, may be wound around the support member 30 and the respective ends of the intermediate portion 603 of the fusible element 60, as shown in FIGS. 1 and 3. The fastening tapes 52 and 54 may be of the electrically insulating type, such as a glass material.

In order to provide the necessaryelectrical connections between the opposite ends of the fusible element 60 and the associated terminal members 72 and 74, the ends of the fusible element 60 may be assembled to pass through the central openings in the associated terminal members 72 and 74 prior to the assembly of the associated end caps 82 and 84 over the ends of the casing 20 and then be clamped or secured between the end caps 82 and 84 and the respective terminal members 72 and 74.

In order to additionally aid or assist in arc interruption during the operation of the fuse structure and to provide the current limiting action which is necessary in a fuse structure of the type described, the space between the casing and the fusible element 60 and between the casing 20 and the supporting member 30 is substantially filled with a finely divided pulverulent or granular arc-quenching material, such as silica sand or quartz sand, in which the fusible element and the support member 30 are effectively embedded. It is to be noted that after the casing 20 is substantially filled with the arc-quenching material which is indicated at 50 in FIG. 1, the arc-quenching material 50 is then preferably compacted by any suitable means, such as vibration or other known methods. It is to be noted in FIG. 2 that where the support member 30 has a central opening or inner bore as indicated at 30A the inner bore is also preferably substantially filled with the same arc-quenching material as indicated at 50 in FIG. 1.

In the operation of the overall fuse structure 10 as illustrated when an abnormal or overload current starts to flow through the fusible element 60 of the fuse structure 10, the intermediate portion 608 having a relatively smaller cross-sectional area will begin to melt initially and a series of arcs and corresponding arc voltages will develop between the particles or drops of vaporized or melted fusible material from which the fusible element is formed. As the melting of the intermediate portion 608 proceeds, the total are voltage which develops in the intermediate portion 60B will increase quickly to a peak value to limit the value of overload current which flows to a value which is less than that available in the electrical circuit which is protected by the fuse structure 10. When the normally solid material from which the support member 30 is formed is exposed to the arcs which result when the fusible element 60 melts or fuses, water vapor is evolved from the support member 30 where the support member 30 is formed from compressed boric acid during an interrupting operation of the fuse structure 10. The water vapor serves so effectively to cool the arcs which result and to provide a turbulent region at the interface between the support member and the associated arc-quenching material 50 that the arcs are quenched or interrupted in a relatively shorter time or with a considerably fewer number of cycles of arcing current than would result if other types of materials were employed to form the support member 30 or if a support member were not provided. It is important to note that during the arcing which results in the operation of the fuse structure 10, the support member 30 is substantially non-tracking to thereby prevent the formation of an axially continuous leakage current path along the support member 30 and the subsequent breakdown of the support member which frequently characterizes known types of support members employing organic types of gas evolving material. It is also to be noted that the ball of low melting point material which may be tin or solder material as previously mentioned causes the fusible element 60 to melt or fuse at a temperature of about 182 to 232 Centigrade. Where the support member 30 is formed from compressed boric acid material, the ball of low melting point material 65 thus promotes the melting of the fusible element 60' at a temperature which prevents damage to the support member 30 due to long time overload conditions to which the fuse structure 10 may be subjected. It has also been found that for a particular thickness of the material from which the fusible element 60 is formed, it has been possible in a fuse structure as disclosed to interrupt minimum melting currents at a relatively high voltage (approximately 8 KV) which is several times that which was previously possible.

It is to be noted that during the operation of the fuse structure 10 as just described, the arc-quenching material 50 which is disposed in the casing will aid or assist in arc interruption by absorbing the thermal energy of the arc currents which develop during the operation of the fuse structure 10 and form a fulgurite with the vaporized material of the fusible element 60 as is well known in the fuse art. In addition, the presence of the arc-quenching material 50 assists in preventing excessive gas evolvement from the support member 30 and excessive internal gas pressures within the casing 20 of the fuse structure 10. It is also important-to note that where the support member 30 is formed from compressed boric acid material, the water vapor which is evolved from the support member 30 in the presence of an are so effectively cools the are to aid in are interruption and so effectively thermally protects the outer casing 20 that no charring or burning of the inner surface of the casing has been found to occur and the easing 20 remains after an interrupting operation in a substantially undamaged condition. The advantage of the protection thus provided by the operation of the fuse structure is that the insulating characteristics of the casing 20 remain substantially unaffected following an arc interrupting operation of the fuse structure I0.

Referring now to FIG. 6 there is illustrated a second electrically insulating support member 200 which may be employed instead of the support member 30 in the fuse structure 10 where desired. More specifically, the support member 200 includes a central supporting rod 240 which is formed from a solid, electrically insulating material which does not evolve gases in the presence of an arc and which is substantially non-tracking in the presence of an arc, such as a ceramic or other inorganic material. The support member 200 also includes a plurality of generally annular or tubular blocks 2110, 220 and 230 which are disposed to surround and extend axially along the central support rod 240, as shown in FIG. 6. In order to control the amount of gas which is evolved during the operation of the overall fuse structure of which the support member 200 forms a part, the axially end portions or blocks 220 and 230 are also formed from an electrically insulating solid material which does not evolve gases in the presence of an arc and which is substantially non-tracking in the presence of an arc such as a ceramic or other inorganic material. In the construction of the support member 200, only the axially intermediate broad or tubular member 210 is formed from an electrically insulating normally solid material which evolves gases in the presence of an arc but which is substantially non-tracking in the presence of an arc, such as the compressed boric acid material as previously described in detail. In an overall fuse structure which includes the support member 200, the associated fusible element or elements would be helically disposed on the support member 200, as previously described in detail in connection with the fuse structure 10. In the operation of an overall fuse structure including the support member 200, the gas evolving material from which the intermediate support portion 210 is formed is particularly beneficial in interrupting relatively low overload currents which would normally cause the axially intermediate portion of the fusible element which is disposed on the support member 200 to melt due to the presence of the ball of low melting point material which would be similar to the ball of low melting material 65 shwon on the fusible element in FIG. I. In other words, in the operation of an overall fuse structure including the support member 200, the structural position of the intermediate support portion 2110 would be particularly beneficial or effective since the gases evolved would be particularly helpful in interrupting relatively low overload currents which would cause the axially intermediate portion of the associated fusible element to initially melt adjacent to the support portion 210 and cause the intermediate support portion 2H0 to evolve one or more gases which aid in arc extinction when subjected to an arc.

It is to be understood that the teachings of the invention may be applied to a current limiting fuse structure in which the support member either includes a central opening as disclosed or may be entirely solid where desired. Where the support member does include a central opening, an additional fusible element such as a high resistance fusible wire may be disposed to pass through the central opening and to cooperate with an associated indicating means where desired. It is to be understood that in the fuse structure disclosed, the sup port member 30 may be modified where desired to form only the axially intermediate blocks such as the blocks 34, 35 and 36 from a gas evolvingmaterial, which is substantially non-tracking, such as compressed boric acid, as previously described, and to form the remaining blocks at the axial ends of the support member 30 from an electrically insulating material which does not evolve gases in the presence of an arc and which is substantially non-tracking in the presence of an arc such as a ceramic or other inorganic material. It is to be further understood that other types of suitable electrically insulating materials which evolve one or more gases in the presence of an arc and which are also substantially non-tracking in the presence of an arc may be employed, where desired in a particular application, to provide various degrees of improvement in the fuse structure as disclosed. Such materials may include highly compressed calcium carbonate which evolves gases in the presence of an are which aid in arc extinction, such as carbon monoxide or carbon dioxide, and which is substantially non-tracking in the presence of an arc.

A current limiting fuse structure embodying the teachings of this invention has several advantages. For example, a current limiting fuse structure including an electrically insulating support member as disclosed which is at least partially formed from a material which evolves gases in the presence of an arc which aid in arc extinction and which remains substantially nontraclting in the presence of an arc provides the advantages of improved arc interruption which results from the evolution of the gases during arc interruption while overcoming or preventing the formation of an axially extending continuous leakage current path along the support member which results in fuse structure including known types of organic gas evolving support members. In addition, the disclosed current limiting fuse structure prevents possible damage to the associated electrically insulating casing which is normally formed from an organic material for reasons of structural strength and other reasons during the interrupting operation of the fuse structure and the possible voltage breakdown which might otherwise result if such damage should occur. A further advantage of the applicants invention is that it permits a relatively much more compact construction of a fuse structure which is particularly adopted for high voltage applications by reducing the required axial dimension of the overall fuse structure. Finally, the current limiting fuse structure as disclosed reduces the time required for the interruption of an arc and also reduces the corresponding damage that might otherwise result in the electrical circuit which is protected by the fuse structure. A further advantage of the disclosed current limiting fuse structure is that it lends itself to the application of relatively high voltage fuses which are adapted to interrupt overload currents over a relatively wide range or full range of overload currents. As previously described, the current limiting fuse structure as disclosed particularly improve the relatively lower overload current interrupting ability of a high voltage current limiting fuse of the type described.

We claim:

ll. A current limiting fuse structure comprising a generally tubular, electrically insulating casing, terminal means disposed adjacent to each of the opposite ends of said casing, an electrically insulating support member disposed in and laterally spaced from the inner bore of said casing and extending axially between said terminal means, a fusible element disposed in and laterally spaced from the inner bore of said casing on said support member and connected between said terminal means, at least the axially intermediate portion of support member on which said fusible element is disposed being formed from a normally solid material which is adapted to evolve a gas which aids in arc extinction in the presence of an arc which results when said fusible element melts, said last-mentioned material being substantially non-tracking in the presence of an arc and a quantity of pulverulent, arc quenching material disposed between said support member and said casing to substantially fill said casing and disposed in contact with said fusible element.

2. The combination as claimed in claim 1 wherein said support member includes axially extending portions at each end thereof formed from a material which is substantially non-gas-evolving when exposed to an arc.

3. The combination as claimed in claim 1 wherein said electrically insulating support member is formed substantially entirely from highly compressed boric acid material.

4. The combination as claimed in claim 1 wherein said last-mentioned material comprises highly compressed boric acid material.

5. The combination as claimed in claim 1 wherein said last-mentioned material comprises compressed calcium carbonate material.

6. The combination as in claim 1 wherein said fusible element is formed from electrically conducting material of the flat ribbon type having periodically spaced restricted portion along its length, the intermediate portion of said fusible element including a plurality of turns helically disposed on said support member.

7. The combination as claimed in claim 2 wherein said fusible element is formed from electrically conducting material of the flat ribbon type having periodically spaced restricted portion along its length, the intermediate portion of said fusible element including a plurality of turns helically disposed on said support member.

8. The combination as claimed in claim 3 wherein said fusible element is formed from electrically conducting material of the flat ribbon type having periodically spaced restricted portion along its length, the intermediate portion of said fusible element including a plurality of turns helically disposed on said support member.

9. The combination as claimed in claim 4 wherein said fusible element is formed from electrically conducting material of the flat ribbon type having periodically spaced restricted portion along its length, the intermediate portion of said fusible element including a plurality of turns helically disposed on said support member. 2

10. The combination as claimed in claim 5 wherein said fusible element is formed from electrically conducting material of the flat ribbon type having periodically spaced restricted portion along its length, the intermediate portion of said fusible element including a plurality of turns helically disposed on said support member. 

1. A current limiting fuse structure comprising a generally tubular, electrically insulating casing, terminal means disposed adjacent to each of the opposite ends of said casing, an electrically insulating support member disposed in and laterally spaced from the inner bore of said casing and extending axially between said terminal means, a fusible element disposed in and laterally spaced from the inner bore of said casing on said support member and connected between said terminal means, at least the axially intermediate portion of support member on which said fusible element is disposed being formed from a normally solid material which is adapted to evolve a gas which aids in arc extinction in the presence of an arc which results when said fusible element melts, said last-mentioned material being substantially non-tracking in the presence of an arc and a quantity of pulverulent, arc quenching material disposed between said support member and said casing to substantially fill said casing and disposed in contact with said fusible element.
 2. The combination as claimed in claim 1 wherein said support member includes axially extending portions at each end thereof formed from a material which is substantially non-gas-evolving when exposed to an arc.
 3. The combination as claimed in claim 1 wherein said electrically insulating support member is formed substantially entirely from highly compressed boric acid material.
 4. The combination as claimed in claim 1 wherein said last-mentioned material comprises highly compressed boric acid material.
 5. The combination as claimed in claim 1 wherein said last-mentioned material comprises compressed calcium carbonate material.
 6. The combination as in claim 1 wherein said fusible element is formed from electrically conducting material of the flat ribbon type having periodically spaced restricted portion along its length, the intermediate portion of said fusible element including a plurality of turns helically disposed on said support member.
 7. The combination as claimed in claim 2 wherein said fusible element is formed from electrically conducting material of the flat ribbon type having periodically spaced restricted portion along its length, the intermediate portion of said fusible element including a plurality of turns helically disposed on said support member.
 8. The combination as claimed in claim 3 wherein said fusible element is formed from electrically conducting material of the flat ribbon type having periodically spaced restricted portion along its length, the intermediate portion of said fusible element including a plurality of turns helically disposed on said support member.
 9. The combination as claimed in claim 4 wherein said fusible element is formed from electrically conducting material of the flat ribbon type having periodically spaced restricted portion along its length, the intermediate portion of said fusible element including a plurality of turns helically dIsposed on said support member.
 10. The combination as claimed in claim 5 wherein said fusible element is formed from electrically conducting material of the flat ribbon type having periodically spaced restricted portion along its length, the intermediate portion of said fusible element including a plurality of turns helically disposed on said support member. 